The present invention relates essentially to a spark-ignited internal combustion engine operated with charge stratification, and, more particularly, to an internal combustion engine including a cylinder head having a main combustion chamber and an auxiliary combustion chamber connected to the main combustion chamber. The auxiliary combustion chamber is formed within a cup-shaped, thin-walled insert positioned in a cavity of the cylinder head, the insert having an opening providing a passage connecting the main combustion chamber and the auxiliary combustion chamber and a pair of openings for receiving an ignition device and a fuel delivery device.
A spark-ignited internal combustion engine operated with charge stratification and described in German Offenlegungsschrift No. 2,327,703 comprises a cylinder head having a cavity accommodating a cup-shaped insert which delimits an auxiliary combustion chamber, the cavity opening away from a main combustion chamber. The cavity also accommodates a valve seat for an auxiliary intake valve which delivers a fuel-air mixture, comparatively rich in fuel content, to the auxiliary combustion chamber. Since the cup-shaped insert also opens away from the main combustion chamber, it must be introduced into the cylinder head from above and maintained therein along with the valve seat. The closed-end of the insert is located substantially adjacent the main combustion chamber and includes an opening aligned with a connecting channel which communicates with the main combustion chamber.
At low operating temperatures (i.e., during the cold-starting and warming-up phases of the engine) the insert is spacedly positioned in the cavity, a distance from the surrounding walls of the cylinder head. At higher operating temperatures produced when the engine is operating at higher loads, the insert expands and contacts the surrounding walls of the cylinder head. Thus, at high operating temperatures, the large quantity of heat produced, especially in the vicinity of the connecting channel communicating with the main combustion chamber, is transferred rapidly from the insert to the surrounding walls of the cylinder head so that the temperature of the insert does not exceed a predetermined maximum temperature.
However, one disadvantage of such an internal combustion engine is that the combustion residues, such as soot and the like, produced especially during the warming-up phase of the engine (i.e., when the engine has not yet attained sufficient operating temperatures), are deposited in the space between the insert and the surrounding walls of the cylinder head. After the engine has been operated for an extended period of time, the deposited residues completely obstruct the space between the insert and the surrounding walls of the cylinder head. Thus, the deposited residues prevent the expansion of the insert, and hence its application against the surrounding walls of the cylinder head. Since the deposited residues are poor conductors of heat (i.e., insulators), the heat generated during combustion in the auxiliary combustion chamber is transferred from the insert to the cylinder head only over the seat surfaces of the insert which contact the cavity in the vicinity of the auxiliary intake valve for the purpose of supporting the insert in the cavity.
However, heat dissipation through the seat surfaces of the insert cannot be sufficient, even if the insert is manufactured from a material having good heat conductivity, since the portion of the insert subjected to the highest temperatures (i.e., the portion in the vicinity of the connecting channel opening) is at the end of the insert furthest from the seat surfaces. Accordingly, in spite of comparatively high structural costs, the known stratified-charge internal combustion engines cannot effectively prevent the insert from exceeding the predetermined maximum temperature which may be attained when the engine is operating at higher load conditions, and hence cannot avoid consequent spontaneous ignitions due to excessive heat. These problems are amplified if the auxiliary intake valve is replaced by a fuel injection nozzle, since the auxiliary intake valve tends to cool the insert through the separate delivery of the fuel-rich mixture to the auxiliary combustion chamber.